1. Field of the Invention
The present invention relates to a feed-forward distortion compensation amplifier and a method of amplifying a signal with feed-forward distortion compensation for use in a repeater for high-frequency radio communications, and more particularly to a feed-forward distortion compensation amplifier and a method of amplifying a signal with feed-forward distortion compensation for suppressing erroneous operation even when distortion control quantities in a distortion detecting loop and a distortion compensating loop are brought out of balance by inputting an instantaneous peak power level is inputted.
2. Description of the Related Art
Repeaters used in mobile radio systems such as portable telephone systems amplify radio waves from a base station and transmit amplified radio waves to mobile stations. In order to simultaneously amplify radio waves in multiple channels, the amplifiers in the repeaters are required to be considerably linear. However, there is a certain limitation on the linearity of the amplifiers in the repeaters.
Some repeaters incorporate a distortion compensation amplifier referred to as a nonlinear distortion compensation circuit, i.e., a SAFF (Self-Adjusting Feed-Forward) circuit, for canceling a distortion due to a nonlinear operation of the amplifier caused when an excessively high signal due to an instantaneous peak power level is applied to the amplifier.
Known feed-forward distortion compensation amplifiers of the above type are disclosed Japanese laid-open patent publications Nos. 5-315847 and 8-78965, for example.
The disclosed feed-forward distortion compensation amplifiers generally have a distortion detecting loop and a distortion compensating loop that are connected in series with each other. In the distortion detecting loop, an input signal is branched into two input signals, and one of the branched input signals is amplified by an amplifier whose output signal is branched. The branched output signal from the amplifier and the other branched input signal are inverted relatively to each other and combined together to extract a distortion component signal produced by the amplifier. In the distortion compensating loop, the distortion component signal outputted from the distortion detecting loop and the output signal from the amplifier are inverted relatively to each other and combined together to obtain an amplifier output signal from which the distortion component signal produced by the amplifier has been removed.
If a CDMA (Code Division Multiple Access) signal having a signal envelope that contains instantaneous peaks P1, P1' as desired waves, as shown in FIG. 8 of the accompanying drawings, is inputted to the above feed-forward distortion compensation amplifier, then the peak power becomes close to or higher than the saturation level, resulting in the generation of a distortion having an extremely high level due to the amplification in the distortion detecting loop.
The above generation of a distortion is illustrated in FIG. 9 of the accompanying drawings. FIG. 9 shows input/output characteristics of a general main amplifier for use in the distortion detecting loop of a feed-forward distortion compensation amplifier. If the gradient of an actual performance line of the main amplifier deviates from an ideal line, then the output voltage is suppressed as it reaches the saturation level, forcing the main amplifier to generate a distortion.
If a signal having an instantaneous peak as shown in FIG. 6(a) of the accompanying drawings is inputted to the main amplifier, then an output signal from the main amplifier is suppressed by 1 dB as shown in FIG. 6(b) of the accompanying drawings, with the result that a canceled distortion quantity is about 18 dB according to the equation (1) shown below.
If the main amplifier operates at all times with a canceled distortion quantity of 30 dB or greater, then, as shown in FIG. 6(c) of the accompanying drawings, the distortion canceling level is lowered due to the instantaneous peak level, allowing a level that is about 12 dB higher than the normal canceled distortion quantity to be inputted to a distortion removing loop.
In the above example, it is known that the canceled distortion quantity is expressed by the following equation (1): EQU Canceled distortion quantity=10 log(1+10.sup.d/10 -2.times.10.sup.d/20.times.COS p)
where d represent the amplitude deviation (dB), and p the phase deviation (deg.).
With the conventional feed-forward distortion compensation amplifiers, if the input signal contains an instantaneous peak, then the amplifiers are unable to perform their normal amplifying operation. If such an amplifier is used in a repeater amplifier unit, then it tends to lower the quality of radio communications.
FIG. 1 of the accompanying drawings shows a feed-forward distortion compensation amplifier according to the present invention. The above shortcoming will be described below with reference to FIG. 1.
In the distortion detecting loop, a vector adjuster 3 is controlled to minimize an output signal from a detector 2, i.e., to minimize a distortion component signal from a main amplifier 1 which is extracted by a canceling process performed by the distortion detecting loop.
When the detector 2 detects a peak power level, the distortion detecting loop starts a control operation to change the vector adjuster 3 in order to keep the distortion detecting loop in balance. However, since the output level of the detector 2 quickly increases because of the inputted peak level, even if the change in the output level of the detector 2 is automatically controlled so as to converge, it takes a certain period of time until the detector 2 returns to its normal control mode, and the distortion detecting loop tends to be unlocked, causing the feed-forward distortion compensation amplifier to operate in error.
The above problems holds true for a feed-forward distortion compensation amplifier which employs a pilot signal as an input signal for distortion detection (corresponding to the embodiment of the present invention shown in FIG. 7 of the accompanying drawings) so that a control process for distortion detection can be performed in the absence of an input signal. If an input signal (communication signal) such as an RF signal to be amplified contains an instantaneous peak, then a control process for distortion compensation cannot properly be performed on the input signal, with the result that desired waves are liable to suffer distortions upon amplification thereof.